nucleus

a thing or part forming the center around which other things or parts are grouped or collected; core

anything serving as a center of growth or development: the nucleus of a library

Anat. a group of nerve cells in the brain or spinal column

Astron. the bright, central part of the head of a comet

Biol. the central, usually spherical or oval mass of protoplasm present in most plant and animal cells, containing most of the hereditary material and necessary to such functions as growth, reproduction, etc.

Bot. the central point in a starch grain

Chem. in organic chemistry, a fundamental, stable arrangement of atoms, as the benzene ring, that may occur in many compounds by atomic substitution without structural change

the central part of an atom, the fundamental particles of which are the proton and (except for hydrogen) neutron: it carries a positive charge and constitutes almost all of the mass of the atom

Phonet. the most sonorous portion of a syllable, usually a vowel

Origin of nucleus

Modern Latin from L, a nut, kernel, for nuculeus, diminutive from nux (gen. nucis), nut

nucleus

A central or essential part around which other parts are gathered or grouped; a core: the nucleus of a city.

Something regarded as a basis for future development and growth; a kernel: a few paintings that formed the nucleus of a great art collection.

Biology A membrane-bound organelle within a eukaryotic cell that contains most of the cell's genetic material. DNA transcription takes place in the nucleus.

Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.

Physics The positively charged central region of an atom, composed of protons and neutrons, about which negatively charged electrons orbit. Extremely small and dense, the nucleus contains almost all of the mass of an atom.

Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.

Astronomy

a. The central, often brightest part of the head of a comet.

b. The solid part of a comet, composed of ice and smaller amounts of dust and rock.

c. The central, often brightest part of a galaxy.

Meteorology A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.

Linguistics The part of a syllable having the greatest sonority. In the word middlemost (mĭd′l-mōst′) the nuclei of the three syllables are (ĭ), (l), and (ō); in the Czech word krk (“neck”), the nucleus is (r).

The positively charged central region of an atom, composed of one or more protons and (for all atoms except hydrogen) one or more neutrons, containing most of the mass of the atom. The strong force binds the protons and neutrons, also known as nucleons, to each other, overcoming the mutual repulsion of the positively charged protons. In nuclei with many nucleons, however, the forces of repulsion may overcome the strong force, and the nucleus breaks apart in the process of radioactive decay. The protons and neutrons are arranged in the nucleus in energy levels known as shells analogous to those of the electrons orbiting the nucleus. The number of protons in the nucleus determines the atom's atomic number and its position in the Periodic Table.

An organelle in the cytoplasm of eukaryotic cells (all cells except prokaryotes) that contains nearly all the cell's DNA and controls its metabolism, growth, and reproduction. The nucleus is surrounded by a pair of membranes called the nuclear envelope, which can be continuous in places with the membranes of the endoplasmic reticulum. The membranes of the nuclear envelope have interconnected pores that allow the exchange of substances with the cell's cytoplasm. The nuclear DNA is wrapped around proteins (called histones) in strands of chromatin, which exists in a matrix known as nucleoplasm (analogous to the cytoplasm outside the nucleus). Just prior to cell division, the chromatin condenses into individual chromosomes, which contain the cell's hereditary information. The nucleus also contains at least one spherical nucleolus, which mainly contains RNA and proteins and directs the construction of the cell's ribosomes.

The solid central part of a comet, typically several kilometers in diameter and composed of ice, frozen gases, and embedded chunks of rock and dust. It is the permanent part of a comet from which the coma and tail are generated as the comet approaches the Sun.

nucleus - Medical Definition

Biology A membrane-bound organelle within a eukaryotic cell that contains most of the cell's genetic material. DNA transcription takes place in the nucleus.

Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.

Physics The positively charged central region of an atom, composed of protons and neutrons, about which negatively charged electrons orbit. Extremely small and dense, the nucleus contains almost all of the mass of an atom.

Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.

Sentence Examples

Starch exists, in the majority of cases, in the form of grains, which are composed of stratified layers arranged around a nucleus or hilum.

He discovered the nucleus of the cell.

Of the surplus 1,000,000 was allocated to the improvement of posts, telegraphs and telephones; 1,000,000 to public works (~72o,ooo for harbour improvement and 280,000 for internal navigation); 200,000 to the navy (~I32,ooo for a second dry dock at Taranto and 68,000 for coal purchase); and 200,000 as a nucleus of a fund for the purchase of valuable works of art which are in danger of exportation.

In the spermatozoids of Chara, Vascular Cryptogams, and in those of Cycas, Zamia and Ginkgo, the cilia arise from a centrosome-like body which is found on one side of the nucleus of the spermatozoid mother-cell.

They are more easily seen, when the nucleus is about to undergo mitosis, at the ends of the spindle, where they form the centres towards which the radiating fibres in.